1. Field of the Invention
This invention relates to a method of manufacturing screws.
2. Description of the Prior Art
Screws which are used inside semiconductor manufacturing apparatuses are put in high vacuum atmosphere. Therefore, if air remains between tip portions of the screws and mated female threads formed in the apparatus when the screws have been fastened, there is a problem in that the remaining air leaks into the inside space of the apparatus which is being kept in a predetermined vacuum condition, thereby lowering the degree of the vacuum.
As for the method for solving this problem, there is known a method that uses a screw having a hollow axial bore extending along its longitudinal direction to evacuate the remaining air at the tip portion thereof through the hollow axial bore.
Normally, the hollow axial bore of the screw is formed by means of drilling or electrical discharge machining.
In the case where a small diameter bore is to be formed by the drilling, a drill having a small diameter corresponding to the diameter of the bore is required. However, since the drilling process requires large power, use of such a small diameter drill involves problems in its strength and durability. In particular, when a long bore is to be formed, an elongated small diameter drill is used. Such an elongated small diameter drill is liable to be broken due to its elongated shape, and it is also liable to be damaged since large vibration likely to occur during the drilling process.
On the other hand, the method using the electrical discharge machining has an advantage in that it is possible to form such a small diameter bore relatively easily as compared with other methods. However, there is a problem in that a machining cost is high, thus leading to increased price of the screw. Further, it is difficult for the method using the electrical discharge machining to form an elongated bore.
Moreover, when a screw is formed of a material which is difficult to be machined, there is a case that such a screw can not be machined by these methods or a case that it takes a relatively long time to machine it.
As stated in the above, the conventional screw manufacturing methods involve problems in that the size of the diameter of a hollow axial bore is limited and in that materials which can be used for manufacturing screws are also limited.
It is therefore an object of the present invention to provide a method of manufacturing screws by which screws having a hollow axial bore having a predetermined diameter can be formed easily with excellent machinability.
In order to achieve the object, the present invention is directed to a method of manufacturing screws which comprises the steps of: (A) producing a rod-shaped green body having a hollow axial bore from a feed stock containing material powder by means of extrusion molding; (B) debinding the green body to obtain a brown body; and (C) sintering the brown body to obtain a sintered compact, wherein at least one machine working including a thread cutting process is carried out at anytime after the completion of the step (A).
According to the screw manufacturing method described above, it is possible to manufacture screws having a hollow axial bore with excellent machinability and productivity as well as high dimensional precision. Further, it becomes possible to manufacture screws having complex shapes and provide screws formed of hard materials that were difficult to be machined by the conventional methods.
Preferably, the machine working is carried out between the step (A) and the step (B) onto the green body obtained in the step (A). In this case, since the machine working is carried out for the green body which is not hard as compared with the finally obtained sintered compact, it is possible to carry out the machine working easily (that is, the machine working can be carried out with good machinability) regardless the composition of the material powder and the kind thereof. Therefore, control of shape and size can be made easily, thereby enabling to improve dimensional precision at the processing portions and machine complex and intricate shapes.
Further, it is also preferred that the machine working is carried out between the step (B) and the step (C) onto the brown body obtained in the step (B). In this case, as is the same as the above case, since the machine working is carried out for the brown body which is not so hard as compared with the finally obtained sintered compact, it is possible to carry out the machine working easily (that is, the machine working can be carried out with good machinability) regardless the composition of the material powder and the kind thereof. Therefore, control of shape and size can be made easily, thereby enabling to improve dimensional precision at the processing portions and machine complex and intricate shapes.
Furthermore, in the present invention, the machine working may be carried out onto the sintered compact obtained in the step (C) after the completion of the step (C). When the machine working is carried out onto such a sintered compact, less changes occur in shape and dimension of the sintered compact during the machine working as compared with the case where machine working is carried out onto a green body or a brown body. In particular, dimensional deviation at the thread portion becomes extremely small, thereby improving dimensional precision.
Moreover, in the present invention, the step (C) can be carried out by diving the step into a plurality of sintering steps. This improves sintering efficiency so as to accomplish the sintering in a shorter time, thereby enabling to improve producticity.
In this case, it is preferred that the plurality of sintering steps includes a step of obtaining a pre-sintered compact by pre-sintering the brown body and a step of obtaining a secondary sintered compact by sintering the pre-sintered compact, in which the machine working is carried out onto the pre-sintered compact. The pre-sintered compact is not so hard as compared with the sintered compact, it is possible to perform the machine working thereon without difficulty (that is, under good machinability) regardless the composition of the powder material and the kind thereof. Accordingly, it is easy to control the shape and size when manufacturing a screw, thereby enabling to improve dimensional precision at the machining portions and to machine complex and intricate shapes. Further, as compared with the case where a green body or a brown body is machined, since the pre-sintered compact has undergone the pre-sintering process, less changes occur in shape and size during the machine working, thus leading to quite small dimensional deviation in the thread portions, thereby improving dimensional precision.
Further, in the present invention, it is preferred that the diameter of the hollow axial bore after the completion of the sintering step is equal to or less than 1.5 mm. This range of the bore prevents the thickness of the shank and tip portions of the screw from being too thin even if the screw is a small diameter screw, thereby ensuring sufficient strength.
Further, in the present invention, it is also preferred that the extrusion pressure of the extrusion molding is equal to or less than 1000 kgf/cm2. This range of the extrusion pressure does not give excessively large load by high temperature and pressure to the extrusion molding machine, thereby enabling to carry out the molding without any trouble.
In the present invention, the material powder is metal powder or ceramics powder. By using these materials, it is possible to obtain screws having heat resistance property and corrosion resistance property.
Further, in the present invention, it is preferred that the porosity of the sintered compact finally obtained is less than 7%. The sintered compact having the porosity of this range has high density, high strength and high dimensional precision, so that not only sintering flaws can be prevented but also good appearance can be obtained. Further, sintering can be carried out effectively to complete the sintering in a shorter time, thereby improving productivity.
Furthermore, the screws manufactured by the method of this invention are preferably used under reduced pressure or vacuum. When the screws are used in such atmosphere, air can be evacuated from the hollow axial bore, so that the degree of vacuum in the chamber in which the screws are being used can be satisfactorily maintained. Therefore, the screws manufactured by the method of this invention are suitably used in chambers of semiconductor manufacturing apparatuses, vacuum deposition apparatuses and spattering apparatuses and the like.
The above described and other objects, structures and advantages of the present invention will be apparent when the following detailed description of the preferred embodiment is considered taken in conjunction with the appended drawings.